Exhaust gases discharged from diesel engines contain particulate matter (PM) comprising as main components carbonaceous soot and soluble organic fractions (SOFs) comprising high-boiling-point hydrocarbon components, and are likely to adversely affect humans and environment when discharged to the air in large amounts. Accordingly, a ceramic honeycomb filter (hereinafter referred to simply as “honeycomb filter”) for capturing PM has conventionally been attached to an exhaust pipe from the diesel engine. FIGS. 1 and 2 show one example of honeycomb filters for cleaning an exhaust gas by capturing PM. The honeycomb filter 10 comprises a ceramic honeycomb structure (hereinafter referred to simply as “honeycomb structure”) comprising porous cell walls 2 forming a large number of outlet-sealed flow paths 3 and inlet-sealed flow paths 4 and a peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6b alternately sealing an exhaust-gas-inlet-side end surface 8 and an exhaust-gas-outlet-side end surface 9 of the outlet-sealed flow paths 3 and the inlet-sealed flow paths 4 in a checkerboard pattern.
This honeycomb filter is required to capture particulate matter contained in the exhaust gas with high efficiency during use, with low pressure loss exerting little load to engines. However, because the more particulate matter captured, the more cell wall pores clogged, resulting in higher pressure loss, the captured particulate matter should be burned off to regenerate the honeycomb filter. Thus, the honeycomb filter is required to have high heat resistance and heat shock resistance, because it is repeatedly exposed to high temperatures while burning particulate matter. Because the burning of more accumulated particulate matter exposes the honeycomb filter to extremely high temperatures, making it likely to partially melt cell walls, the melt-down resistance of the honeycomb filter should be taken into consideration.
Though cordierite is generally used as a material for porous cell walls, cell walls made of cordierite have as low a thermal expansion coefficient as about 10×10−7/° C. Thus, despite excellent heat shock resistance that makes cracking due to heat shock less likely, they are likely to be partially molten when the honeycomb filter is exposed to extremely high temperatures by burning the highly accumulated particulate matter. To improve resistance to melting, it is effective to provide porous cell walls with low porosity for higher thermal capacity, and to use more heat-resistant materials such as silicon carbide, aluminum titanate, etc. However, silicon carbide has a large thermal expansion coefficient and is extremely expensive. Aluminum titanate is decomposed to TiO2 and Al2O3 in a temperature range of 1000-1200° C.
As a cordierite ceramic honeycomb having heat resistance improved while keeping heat shock resistance, JP 60-2270 B discloses a cordierite ceramic honeycomb made of cordierite as a main component, and having a crystal phase containing 2-15% of at least one selected from the group consisting of spinel, mullite and corundum, describing that this ceramic honeycomb has a thermal expansion coefficient of 22×10−7/° C. or less in a temperature range from 25° C. to 1000° C., a softening shrinkage ratio of 10% or less at 1450° C., and a mean pore diameter of 3-30 μm. As a method for producing this cordierite ceramic honeycomb, JP 60-2270 B discloses a method of preparing a batch comprising starting material powders having a chemical composition comprising 42-52% of silica, 34-48% of alumina and 10-18% and magnesia, and at least one crystal selected from spinel, mullite and alumina, plasticizing and forming it to a honeycomb shape, drying and sintering it.
However, because the ceramic honeycomb described in JP 60-2270 B is used for carriers for exhaust-gas-cleaning catalysts for automobiles, it is not produced by such a method as using, for instance, a pore-forming material for high porosity. Accordingly, it has low porosity, and does not have sufficient pressure loss characteristics necessary for ceramic honeycomb filters.
JP 2002-530262 A discloses a ceramic product comprising a crystal phase comprising 65-95% by weight of a first cordierite crystal phase, and 5-35% by weight of a second crystal phase selected from mullite, magnesium aluminate, spinel and sapphirine, having a composition comprising 32-51% by weight of SiO2, 35-49% by weight of Al2O3 and 7-16% by weight of MgO, and porosity of about 20% or more and a thermal expansion coefficient of about 15.0×10−7/° C. or less in a temperature range of 25° C. to 1000° C.
The ceramic honeycomb described in JP 2002-530262 A is also used as carriers for catalysts for cleaning exhaust gases of automobiles, having porosity of substantially about 25-40%, but not having sufficient pressure loss characteristics necessary for ceramic honeycomb filters.
Though it would be possible to provide the ceramic honeycomb filters described in JP 60-2270 B and JP 2002-530262 A with improved pressure loss characteristics by using, for instance, pore-forming materials, etc. for higher porosity, the higher porosity would result in lower thermal capacity, providing the honeycomb filters with insufficient resistance (heat shock resistance and heat resistance) when repeatedly exposed to rapid temperature elevation and high burning temperatures during filter regeneration, so that the honeycomb filters may be molten. In addition, sufficient particulate-matter-capturing performance and strength, characteristics contradictory to low pressure loss, cannot be achieved.